In modem complex flow systems, such as a hydronic heating system for a building, hot water or heating fluid is pumped from a central boiler to various zones. After the heat is dissipated from each zone through terminal devices, the zones are combined to form a single header. In each zone it is important to have a constant flow that does not vary with changes in water pressure throughout the system. Such changes may be caused by opening or closing of valves, changes in pumping pressure, or due to other controls. Thus, unless flow regulation is provided, the system tends to get out of balance and the flow in individual zones can vary randomly. Current methods of flow control are costly and complex. Common practice in the art is to use balancing valves. A balancing valve has an adjustable orifice with a differential pressure tap. Adjustments are made using a spindle calibrated with a read out scale. A differential pressure gauge is connected across the downstream and upstream taps. An example of such a device is found in U.S. Pat. No. 4,672,728 to Nimberger. The pressure drop is correlated with the valve setting to indicate flow rate. Typically, one of the balancing valves is installed in each zone of a hydronic system. The balancing process is further complicated because of the possibility that balancing one zone may change the balance in the other zones. Thus, an iterative process of balancing and rebalancing must be done. This balancing is normally done in the field and usually requires expert service and is also time-consuming.
Although improvements have been made to the balancing valves, little success in reducing the work needed to balance the system has been achieved. Flow controllers today generally all have some type of differential pressure sensing. A differential pressure is sensed across a restriction and is used to drive a diaphragm or a piston or to actuate another valve. Most designs involve complex flow passages and fine orifices that tend to become clogged during operation.
Examples of these types of valves are found in U.S. Pat. No. 3,086,548 to Galiger et al., U.S. Pat. No. 4,893,649 to Skoglund, U.S. Pat. No. 4,959,990 to Morris, and U.S. Pat. No. 5,301,713 to Skoglund.